1. Field of the Invention
The present invention relates to a method of and an apparatus for controlling fuel injection rate in an internal combustion engine. More particularly, the present invention is concerned with a method of and an apparatus for controlling fuel injection rate in an internal combustion engine on the basis of a basic fuel injection period which is determined in accordance with the intake pressure and speed of the engine.
2. Description of the Related Art
In the field of fuel injection-type internal combustion engines, fuel injection rate controlling systems have been known as having steps of detecting the intake pressure and the speed of the engine, computing a basic fuel injection period in accordance with the intake pressure and the engine speed, determining the fuel injection period by correcting the basic fuel injection period in accordance with factors such as the intake air temperature, cooling water temperature, and so forth, and allowing a fuel injector to open for a period of time equal to the thus determined fuel injection period.
In this known system, the intake pressure is picked up by means of a diaphragm type pressure sensor which is attached to the intake pipe of the engine. The output from the pressure sensor is processed by a filter having a time constant of 3 to 5 msec for eliminating the pulsation component of the intake pressure caused by the operation of the engine. The basic fuel injection period is computed from the thus detected intake pressure and the engine speed which is sensed by a suitable engine speed sensor.
This known system has a drawback in that the detected change in the intake pressure has a certain time lag behind the actual change in the intake pressure during a transient period of engine operation, e.g., acceleration, due to a delay of response of the diaphragm of the pressure sensor and due to a delay of response attributable to the time constant of the filter. For instance, when the engine is being accelerated quickly by a quick opening of the throttle valve accompanied by a drastic rise in the intake air pressure, the detected intake pressure rises rather slowly, whereby the basic fuel injection period is computed on the basis of the intake pressure which is lower than the actual intake pressure. In consequence, the air-fuel mixture supplied to the engine becomes too lean, with the result that the response of the engine to the acceleration demand is impaired and noxious exhaust emissions are increased. Conversely, when the engine is being decelerated with the throttle valve closed quickly accompanied by a rapid reduction in the intake pressure, the basic fuel injection period is computed on the basis of the intake pressure which is higher than the actual intake pressure, with the result that the drivability of the engine is impaired due to the supply of a too rich air-fuel mixture, as well as increased noxious exhaust emissions. In order to obviate these problems attributable to the generation of a too rich or a too lean mixture, various corrections are conducted by, for example, employing acceleration increment or deceleration decrement of the fuel supply. As a matter of fact, however, it has been impossible to control the air-fuel ratio of the mixture to command levels over the entire range of the engine operation, because of the presence of the above-mentioned time lag or delay in the detection of the intake pressure in transient periods of the engine operation.
In order to eliminate any time lag in the detection, Japanese patent application Laid-Open No. 28031/1984 proposes, as a parameter of determination of the basic fuel injection period, the amount of opening of the throttle valve of the engine which inherently does not have any time lag to the change in the intake pressure. Thus, a fuel injection rate controlling system proposed by this known art is to compute the basic fuel injection period in accordance with the amount of the throttle opening and the engine speed.
In another known fuel injection rate controlling method proposed in Japanese patent application Laid-Open No. 39948/1984, values of the intake pressure are stored in a table in relation to the throttle opening and the engine speed, and the intake pressure read from the table is used as the base of computation of the fuel injection rate, after a correction of the intake pressure in consideration of the partial pressures in the exhaust gas in exhaust gas recirculating mode in accordance with a signal derived from a pressure sensor.
It is to be understood that a throttle valve is usually disposed upstream from the pressure sensor and, needless to say, upstream from the combustion chamber of the engine. In consequence, a time lag is inevitably caused because certain periods of time are required for the air-fuel mixture to flow from the position of the throttle valve to the position of the pressure sensor and to the combustion chamber. It is also to be understood that the phase of operation of the throttle valve is ahead of the phase of the change in the actual suction of the mixture by the engine, because of the volume of the space in the intake pipe between the throttle valve and the intake valve of the engine. As a consequence, the phase of the intake pressure P(TA, NE), determined in accordance with the amount of throttle opening and the engine speed, is ahead of the phase of the actual intake pressure P, as shown in FIG. 3. At the same time, as will be seen from FIG. 4, the basic fuel injection rate TP (TA, NE) determined by the throttle opening degree and the engine speed is greater than the actually demanded fuel injection rate because the phase of the change in the amount of throttle opening is ahead of the phase of change in the rate of supply of the mixture to the engine. Therefore, when the fuel injection rate is controlled on the basis of the amount of throttle opening and the engine speed, the actual fuel injection rate exceeds the demanded rate during the acceleration so as to make the mixture excessively rich. Conversely, during the deceleration, the actual fuel injection rate becomes smaller than the demanded rate to make the mixture excessively lean. When an acceleration increment of the fuel supply is conducted, the fuel supply rate is increased as hatched in FIG. 4, and cannot eliminate the undesirable effect caused by the above-described phase advance.